An electric power steering apparatus, loaded on a vehicle, such as an automobile, for assisting the operation of a steering wheel (steering of steered road wheels), has been known. The electric power steering apparatus, for example, has an electric actuator device such as a motor, and allows the electric actuator device to generate a steering assist force for a steering mechanism. Concretely, when the steering wheel is rotationally operated (steered) by a driver, the electric power steering apparatus actuates the motor to generate a steering assist force, which is conformed to the steering state of the steering wheel, in a rotating direction. In this manner, a steering torque which the driver requires for steering the steering wheel is reduced.
If the driver releases his or her hands from the steering wheel when rotating the steering wheel, the steered road wheels are about to return to a neutral position (straight travel direction) under a self-aligning torque. If, on this occasion, the above steering assist force in the rotating direction has been generated, the steered road wheels may be inhibited from returning to the neutral position.
Under these circumstances, a proposal has been made for an invention in which the steering assist force in the rotating direction is restricted depending on the steering state of the steering wheel (see Patent Document 1). Patent Document 1 describes that a steering assist command value is determined based on a steering torque and a vehicle speed, the electric motor is controlled according to this value and, at this time, the steering assist command value is restricted (the steering assist force in the rotating direction is restricted) in accordance with the steering state. Concretely, the patent document describes that in a region where the steering torque is great, the steering assist command value is not restricted based on the judgment that the steering wheel is being operated (rotated), whereas in a region where the steering torque is small, the steering assist command value is restricted based on the judgment that the steering wheel is in a return state where the hands have been released from the steering wheel.
By so restricting the steering assist command value, as appropriate, the steered road wheels become easily returnable to the neutral position in a steering wheel return state where the hands have been released from the steering wheel.    [Patent Document 1] JP-A-2009-196573
With the invention according to Patent Document 1, however, even in a steering wheel return state (a state where the hands have been released from the steering wheel), the steering assist force in the rotating direction continues to occur. Even in the invention described in Patent Document 1, therefore, there is a possibility that the return of the steered road wheels to the neutral position will be delayed.
The present invention has been accomplished in the light of the above circumstances. It is an object of the present invention to provide an electric power steering apparatus which can return the steering wheel and the steered road wheels to the neutral position at an earlier time if the driver releases his or her hands from the steering wheel when rotating the steering wheel.
One aspect of the present invention, which solves the above-mentioned problems, is an electric power steering apparatus comprising: an electric actuator for generating a steering assist force for a steering mechanism of a vehicle; and a control device that controls the steering assist force generated by the electric actuator in accordance with a steering operation of a steering wheel by a driver, wherein the control device comprises a rotating assist force computing unit that computes an assist force in a rotating direction of the steering wheel in accordance with a steering state of the steering wheel, a returning assist force computing unit that computes a returning assist force, which is the steering assist force in a direction opposite to the rotating direction, in accordance with the steering state of the steering wheel, and a summing unit that adds up the computed assist force in the rotating direction and the computed returning assist force.